Hair predominantly comprises certain polypeptide chains that are held together by disulfide bonds that link adjacent polypeptide chains. These bonds, which are formed from two cysteine amino acid residues on the adjacent keratin polypeptides, impart to the hair its mechanical strength and extensibility. Exposure to the sun tends to cause breakage of these disulfide bonds predominantly on the outer surface of the hair, including the outer surface of the hair cuticle. This results in stiffness and brittleness of the hair in dry weather and in frizziness of the hair in humid weather. It also results in the hair losing its color and luster. In addition, the breakage of the disulfide bonds of the keratin causes a protective layer of the hair to be broken down, thereby resulting in important constituents of the hair being extracted from the hair during washing.
Sun damage to hair has been controlled by utilizing sunscreens which, when deposited on the hair, absorb ultraviolet light that would otherwise be absorbed by the hair itself. Sunscreen containing compositions, however, tend to be very oily, with the result that they typically are not esthetically appealing. Moreover, many commercially available sunscreens are not effective to shield the hair from all the ultraviolet light in sunlight. For example, a formulation of paraaminobenzoic acid (PABA), when it is deposited on the hair, typically might absorb only about 45 percent of the incident UVB ultraviolet radiation and virtually none of the radiation in the UVA region.